1. Field of the Invention
The present invention relates to an X-ray computed tomography apparatus including an X-ray tube and multi-slice X-ray detectors.
2. Description of the Related Art
A significant challenge for recent X-ray computed tomographic imaging apparatuses is to realize both an improvement in image quality and a decrease in dosage. Image quality improves as the intensity of X-rays with which an object to be examined is irradiated decreases, and vice versa.
In a conventional apparatus, the intensity of X-ray generated during a helical scan is maintained at a constant value. Recently, however, techniques of changing the intensity of X-rays in-accordance with the X-ray transmission factor which changes in accordance with a region of the object have been proposed. In many of these proposals, the X-ray intensity is changed in accordance with a value at a given point on a scanogram. As is known, a scanogram is the two-dimensional intensity distribution of transmitted X-rays which is acquired for a scan-plan.
To acquire scanogram data, an X-ray tube 10 is fixed at a given rotational angle, as shown in FIGS. 1A and 2A. A table top 2a is moved at a constant velocity. During this period, signals are repeatedly read from an X-ray detector 11 at a predetermined cycle.
This X-ray intensity control is effective in a single-slice scan shown in FIG. 1B. As shown in FIG. 2B, however, X-ray intensity cannot be satisfactorily optimized for a multi-slice scan (also called a volume scan). The biggest reason for this is that in a multi-slice scan, data are acquired at once in a wide range with a slice width T2 larger than a slice width T1 in scanogram data acquisition.